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Tomorrow’s urban heat islands

Big-city effects on local temperature could exceed those of global warming

With nearly a million residents, the Tucson metropolitan area is the largest in southeast Arizona. (Wikimedia photo by Zereshk.)

August 15, 2012 | As the world’s largest urban areas grow in size and importance, they are warming more quickly than surrounding areas. That disparity will likely increase, even as cities take steps to reduce their heat island effects, according to a new study by researchers at the Arizona State University and NCAR. The findings appear in the August 12 issue of Nature Climate Change. Led by ASU’s Matei Georgescu, the research team includes NCAR’s Jimy Dudhia.

The authors note that urban expansion worldwide by 2030 is projected to cover a large area, with estimates ranging anywhere from the size of California to the size of the United States. Their analysis focuses on Arizona’s Sun Corridor, a cluster of cities that stretches from Phoenix southeastward through Tuscon to the Mexican border. Now with roughly 5.7 million residents, the corridor is projected to have 8–10 million people by 2040. Many other large urbanized zones around the globe are expected to grow at similar rates.

In major cities, temperatures are typically boosted by several factors. For example, less vegetation means less evaporation and cooling from plants. More buildings and pavement tend to store incoming solar radiation by day and inhibit heat from escaping back out into space at night.

In their Sun Corridor analysis, the authors note that average temperatures at a rural Arizona station rose about 1°C (1.8ºF) from the 1950s to the late 1990s, whereas the Phoenix airport warmed about 3°C (5.4ºF). The airport’s warming was stronger at night than by day; as a result, the average day-to-night temperature spread was reduced by about 3°C.

The ASU/NCAR team used four sets of simulations with NCAR’s advanced research version of the Weather Research and Forecasting model, to compare the temperature effects of an expanding urban heat island with those from overall climate change due to human-produced greenhouse gases.

If those gases increase at a more modest pace than now, and Sun Corridor growth is on the high end of projections, then the local temperature boost from urbanization could be up to several times greater than that from overall climate warming.

If greenhouse gases continue to increase at current rates, or if the corridor grows less quickly than expected, then urbanization effects on temperature may be less than those of global warming.

Reflecting more of the Sun’s rays by painting roofs white (“cool roofs”) could reduce the amount of heat-island warming by up to 50%, the authors found. However, they note, such techniques may do little to reduce the disproportionate warming of nights versus days.